Variable resistor



Jan. v24, 1950 T, B. G1555 ET AL 2,495,321

vARIABLE REsIsToR Filed Jan. 20, 1945 2 SheeS-Sheet l jll 57 i www 47 l? w INVENTORS THOMAS En. GBBEJ GEORGE W. GLMAN GORDON F. L AlNG Patented Jan. 24, 1950 UNITED STATES PATENT OFFICE VARIABLE RE SISTGR Thomas E. Gibbs, Delavan, George W. Gilman, Janesville, and Gordon F. Laing, Delavan, Wis., assignors to The George W. Borg Corporation, Chicago, Ill., a corporation of Delaware Application January 20, 1945, Serial No. 573,680

12 Claims. 1

The present invention relates in general to variable resistors, or potentiometers, but more in particular to variable resistors in which the resistance element has the form of a compound helix, and the object of the invention is to produce a new and improved variable resistor of this character.

According to a feature of the invention, the resistance element is of bare resistance wire helically wound on a core which itself has the form of a helix, and the turns of the wire and the turns of .the core are partially imbedded in the interior Wall of a molded casing which holds the turns in spaced relation.

According to another feature of the invention, a new and improved process of molding is provided, whereby the casing is molded in situ of a heat setting plastic material and whereby the material is prevented from flowing to the interior of the helix in the molding operation, thus keeping the resistance wire clean on the inside of the helix where it is engaged by the wiper or contact member of the resistor.

The foregoing and other features of the invention will be described fully hereinafter, reference being had to the accompanying drawings, in which- Fig. 1 is a vertical section through a variable resistor or potentiometer embodying the invention;

Fig. 2 is a transverse section taken on the line 2-2, Fig. 1;

Fig. 3 is a transverse section taken on the line 3-3, Fig. 1;

Fig. 4 is a view, partly in section, which shows the construction of the resistance element; and

Fig. 5 is a sectional View of a suitable mold with which the process of molding the casing may be carried out.

Referring to the drawings, the variable resistor or potentiometer therein shown comprises a casing I9, having an end closure or cap II, a resistance element I2 in the form of a compound helix, and a rotatable slider or contact member I3 which is adapted for travel along the resistance element to make contact with the successive turns thereof. The resistance element I2 is provided with terminals I4 and I5 and the slider or contact member I3 has a terminal I6. When the device is used as a potentiometer a source of electrical potential is connected to the terminals I4 and I5 and a variable potential is delivered at the terminal I6 depending on the position of the slider I3, as is well known. When used as a variable resistor connections are made to terminals -lll 2 I5 and I4, or to terminals I6 and I5, and the slider I3 is rotated in one direction or the other to vary :the resistance between the connected terminals in known manner.

The resistance element I2 comprises a core I1, Fig. 4, having an insulating coating I8 of suitable insulating material such as Formvar or Formex, and a helically wound resistance wire I9. In the device shown, the core is of No. 14 copper wire, and the resistance wire has a diameter of 2.18 mils. The resistance wire is wound on the core in spaced turns, while the core is straight, and the ends of the winding are secured by soldering a few turns together as indicated at 2D. This prevents the coils from unwinding. The insulated core with the resistance wire wound thereon is then formed into a helix by winding it on a suitable mandrel having a helical groove therein. In the winding operation the relatively soft copper core is given a permanent set and is removed from the form by unscrewing it in the manner of removing a nut from a bolt.

As illustrated in Fig. 1 of the drawings the core has somewhat more than ten complete turns and the terminals I4 and I5 are 3600 degrees apart, measured in terms of the rotation of the slider or contact member I3. The number of turns in the core helix may be varied, however, as also the size of the core and the size of the resistance wire, in order to Vary the ohmic resistance of the resistance element, or for other reasons. The total ohmic resistance of the resistance element shown and described is about 20,000 ohms between terminals.

A suitable mold for making the casing Ill is illustrated inFig. 5 and comprises the base plate 2I, the core 23, the mold 22, the cylinder 25 and the piston 26. The mold is of the extrusion :type and its construction conforms generally to the construction of similar molds which are in cornmon use. It will not be necessary therefore to give more than a brief description of the several parts,

The core 23 has a cylindrical base 21 which is secured to .the base plate 2| by means of screws such as 28. The mold 22 rests on the base plate 2I and has a cavity which conforms t0 the outside dimensions of the casing I0. The mold is properly located on the base plate by means of dowel pins such as 29. The base 2'I oi the core 23 closes the mold cavity at the bottom and the remainder of the core extends into the cavity where it denes the inside wall of the casing. It will be noted that the core includes a cylindrical section 30 Which has a helical groove 3I. The

upper end of section 30 is an annular inclined plane L13 the ends of which are joined by the vertical abutment 44. The reference character 24 indicates an insert which is slidable in the mold 22 and which is attached to the base 2 of the core by means of dowel pins 32 and 33. This insert provides a support for the terminals I4 and I5.

The cylinder 25 closes the mold cavity at the top and has one or more gates such as 34 which lead from the interior of the cylinder to the mold cavity. The cylinder has a centrally disposed recess at the bottom for receiving the bearing member 35, which is held in place by the spring pressed pin 35. The piston 26 has a tapered slot 3l at the end.

It will be understood that means is provided for heating the mold, but since heating means suitable for this purpose is well known, the showing thereof has been omitted in order to avoid complicating the drawings.

The operation ci" molding a casing will now be described, it being assumed for this purpose that the several parts of the mold are disassembled.

As the rst step in the operation a prepared resistance element l2 in the form of a compound helix is taken and is screwed onto the grooved section 3d of the core 23. The turns of the helix are slightly smaller in diameter than the section 3? and the core l? of the helix has suilicient resiliency to make the helix hug the core and follow the groove 3l as it is screwed on. The groove has a depth equal to about one-half the diameter of the helix core Il' and the turns of the groove are spaced apart so as to separate the turns of the helix and hold them in spaced relation as shown.

The terminal l5, which may be already prepared, has a flexible metal strip 38 soldered to it at one end. The free end of this strip is now soldered to the resistance wire on the upper turn of the helix at a point about an inch or so from the end. The strip 38 overlaps a number of turns of the resistance wire and all these turns are soldered together and to the strip. At this time the strip 38 is straight and together with the terminal l5 extends upward parallel to the axis of the core 23. The terminal I4 is arranged similar to terminal l5 and has a flexible metal is p1 ace d around the Core While it is being re strip 3S similar to strip 38 which is soldered to the resistance wire a short distance from the end of the last turn oi the helix. As previously nienticned the helix has somewhat more than ten complete turns, preferably about ten and a quarter turns. A suitable guide may be used to facilitate the correct angular location of the terminals and strips when the latter are soldered in place. The terminals may be in Vertical alignment, as indicated in Figs. l and 5, but if a full 3600 degree range is essential the terminals are spaced apart angularly by a few degrees, or enough to insure that the short-circuited turns of resistance wire at one end of the helix do not overlap those at the other end. Fig. 2 illustrates this in the case of terminal l, which is angularly displaced to the left, it being understood that terminal I5 is similarly displaced to the right.

Having attached the terminals as described, the operator now applies a covering 4G to the grooved section 3G of the core and the helix I2 supported thereon. This covering consists preferably of several layers of a suitable textile fabric such as linen which is thoroughly coated and impregnated with a heat setting plastic material.

This material may be the same kind of material that is to be used for the casing. Bakelite has been used with good results. The covering may be applied by taking a strip of the coated and impregnated fabric cut on the bias, and wrapping it around the helix to form a cylindrical sleeve, then applying a few turns of silk thread and tieing it to hold the sleeve in place. Preferably, however, a number of such sleeves are prepared in advance by wrapping suitable strips of the coated and impregnated fabric on a mandrel and tieing them with thread. With a supply of such sleeves on hand, the operator covers the helix by taking one oi" the prepared sleeves and slipping it into position around the section 3U of the core, as shown in Fig. 5.

The operator now bends the metal strips 38 and 3a around the ends of sleeve 40 so as to bring the terminals into approximately the positions in which they are shown and then places the mold insert 24 in position on the core 23. To eiiiect this operation the dowel pins 32 and 33 are started into the corresponding holes in the core base 2l and at the same time the terminals are started into the holes in the insert, after which the insert may be pressed into position against the core base. If the terminals I4 and l5 ail to fully enter the holes in the insert they may be pressed in with the aid of a pair of pliers or other instrument.

The operator now takes a couple of washers 4l, made of the same material as the sleeve 40, and places them in position on the end of the core as shown.

As mentioned before, suitable heating means is provided for the mold and parts thereof including the core 23. The mold 22 and cylinder 25 may be kept hot, but it is convenient to cut olf the supply of heat to the core 23 while the operations just described are being performed. The core will cool ofi somewhat, therefore, and will have to be re-heated. The heating means may be a hot plate, or an electrical heating element embedded in the base plate 2l for instance. Whatever the means employed may be, the necessity for heating the core may be taken advantage of for partially curing the sleeve ill and the washers 4i, which otherwise would require a separate operation. To attain this object a hood heated, and the heat being confined, the temperature inside 'the hood is raised sufliciently to bring about the desired partial curing of the sleeve and washers, or rather of the Bakelite material with which they are impregnated.

The heated core may now be placed in position on the bed of the press and the mold 22 may be lowered into position as shown, being properly located with respect to the core by the dowel pins such as 2S. rFhe cylinder 25, with the bearing member 35 inserted therein, is then placed on the mold. The cylinder should be so oriented with reference to the mold and core that the gate 34 is not directly above the terminal l5, to avoid danger of breaking the strip 38 by the inowing Bakelite material.

A preheated cake d2 of Bakelite may now be placed` in the cylinder 25, after which pressure is applied by of the piston 26 in the usual manner. The Bakelite is reduced to a plastic condition and flows into the mold through the gates 34, lling the mold cavity and forming the desired casing. The heat and pressure should be maintained for a sufficient length of time,

usually about 3 minutes, to fully cure or set the material.

When the plastic material starts to now into the mold cavity it rst fills the space above the washers 4l, but as soon as pressure begins to build up the washers are bent downward around their edges and the plastic material flows down to ll the rest of the mold cavity. These washers seal the end of the sleeve il and prevent the entrance of the free flowing plastic material into the sleeve. As soon as themold cavity becomes lled the pressure builds up to a high Value and the heat and pressure softens the Bakelite material with which the washers and sleeve arek impregnated. The washers and the sleeve thus become bonded to the rest of the material and form a lining for the casing which under the great pressure applied is caused to conform exactly to the core and to that portion of the resistance element which is exposed outside of the groove 3l. Looking at it another way, the main turns of the resistance element and the individual turns of the resistance wire become embedded in the lined casing, which lls the spaces between the turns and the groove 3| beyond the ends of the helix, whereby the resistance element becomes firmly fixed in the casing as it is formed.

After the casing has become cured and set the mold may be removed from the press and taken apart. The cylinder 25, with the piston 25 retained therein, is first separated from the mold 22. This may be accomplished by driving wedges between the cylinder and mold to break off the sprues in the gates 35i, after which the cylinder may be lifted off, the bearing member 55 remain ing with the casing in which it is embedded. The sprues break where they have the smallest diameter, which is next to the casing. The core 23 may now be removed from the mold by use of a xture comprising a plate similar to the plate 2i having three pins therein corresponding to the dowel pins 23 but somewhat smaller in diameter. The mold is laid on its side and the ixture is applied to the open end with the pins in the holes occupied by the dowel pins. A few blows with a hammer will now drive the dowel pins and the core out of the mold. The insert 25 comes out with the core, to which it is attached by the dowel pins 32 and 33. With the core out of the mold the insert Zit may be pulled off and the completed casing may then be unscrewed from the core. When the piston 25 is removed from the cylinder 25 the residue of molding material, now formed into a Bakelite disc with attached sprues, comes out with the piston from which it can readily be detached by a blow with a hammer. The apparatus is now ready for molding another casing.

The process as described is a hand molding process, but it will be understood that in practice known arrangements for manipulating the mold and parts thereof with the aid of the press will be employed. Each mold should be provided with several cores so that while one core is in the mold other cores may be in course of preparation. Three cores may be used, for example, in a three stage process, comprising an assembly stage, a core heating and sleeve curing stage, and a molding stage.

An essential feature of the process is the use of the sleeve with the associated washers lll, which prevent the free flowing Bakelite from entering between the turns of the resistance wire into the groove 3i of the core and coating the turns of resistance wire on the inside of the helix,

where they are engaged by the slider i3. The Bakelite impregnated fabric of which the sleeve is composed readily enters between the turns of the core wire il as far as the core of the mold, but the spaces between the resistance wire turns in the groove 3l are very small and the fabric of the sleeve cannot enter them. These spaces are thus effectively sealed olf by the sleeve and entrance of the Bakelite into the groove 3| is prevented. This result is promoted by the partial curing of the sleeve and washers prior to the actual molding operation.

Referring now to Fig. 1 again, after the casing lli has been molded as described, it is placed in a lathe and the bearing member 35 is turned down to the proper dimensions, the hole for the shaft l5 is drilled, and the threads are cut for the nut fili. The casing and resistance element are now ready to be assembled with the other parts to complete the variable resistor or potentiometer.

rThe cap li, like the casing l, is a Bakelite molding and is of the proper size to t into the open end of the casing, as shown in Figs. 1 and 3. The cap has a centrally disposed metal insert 41 which is drilled and finished after the molding operation is completed. This operation is carried out by any suitable and known process and need not be described. Y

The terminal I5, on the outside of the cap H, is secured in place by means of a rivet 48. This rivet also serves to hold the contact spring 49, located on the inside of the cap, and conductively connects the terminal with the contact spring. The contact spring is bifurcated and carries the two contacts 5u and 5l, as shown in Fig. 3. Also shown in Fig. 3 is the stop 52 which is formed integrally with the cap. The stop has the same height as the wall on which it is formed.

The shaft i5 is rotatable in the bearing member 35 and has an axial bore at the left hand end to receive the cylindrical end of the lead screw 56. At the other end the lead screw has a cylindrical section 5'! which is located in the hole drilled in the metal insert :il of the cap l l. The lead screw is thus supported on the cap Il at one end and on the shaft d5 at the other end.

The cylinder 58, preferably made of brass, is supported on the two discs 62 and 54, which are made of suitable insulating material such as micarta. The disc 52 is rotatable on the part 5l of the lead screw, while the disc 54 is rigidly fixed to the shaft 45. The lead screw 56 carries the nut 59 which includes a support 60 for the slider or contact spring I3. This support projects through the longitudinal slot 6l in the cylinder 58.

The shaft 45, lead screw 56, and cylinder 58 are assembled outside the casing lli. The disc 54 is rst placed on the shaft against the flange 53 and is secured by staking. The cylinder 5B is then placed in position on the disc 54 and the end of the cylinder is turned in by a spinning operation, which firmly secures the cylinder to the disc. The lead screw 56 with the nut L59 threaded thereon may now be introduced into the cylinder, with the end section 55 inserted in the bearing opening in shaft 45. The disc 62 is then placed on the section 51 of the lead screw and is pressed into the end of cylinder 58. At this point the cylinder and shaft should be tried for end play on the lead screw, holding the disc 62 against the internal shoulder formed in the cylinder 53. The lead screw has shoulders formed at the opposite ends of the threaded section, the end sections 51 and 55 being of reduced diameter, and one of these shoulders is adapted for engagement by the disc 62 and the other by the end of shaft 45. There should be a small amount of end play, sufficient to insure free rotation of the cylinder. To facilitate the manufacturing operations and to insure that another operation will not have to be performed on the lead screw to shorten the distance between the shoulders the parts are preferably so designed and proportioned that ordinarily there will be too much end playv rlhen during the assembling operation if too much end play is found the excess is taken up by means of one or more small washers placed on the lead screw at one end.

Having tested for end play and having made the necessary adjustment, if any was required, the operator inserts the disc 63 into the end oi cylinder 58 on top of the disc 62 and then performs another spinning operation which turns in the end of the cylinder and secures it to the discs S3 and 62. The disc 63 is a contact member, adapted to be engaged by the contacts 50 and I on contact spring 49, and is preferably made of silver, or is silver plated.

The Contact spring I3 may now be mounted on the support 6G by means of a small screw as shown in Fig. l. At one end the spring I3 is provided with a contact B4, Fig 2, and at the other end it is formed into a loop 65 which extends through the slot 6I in cylinder 58 and out again. The slot is necessarily made somewhat wider than the support to insure that the support will be freely movable, but the play which would otherwise result from this construction is taken up by the spring, which holds the support against one side of the slot. are made of suitable insulating material and may be of molded Bakelite, for example. No filler should be used.

The conta-ct spring E 3 is conductively connected to the cylinder by means of a flexible braided conductor 65 which has one end soldered to `the contact spring I3 and the other end soldered to the cylinder 58. The conductor 5B should be attached to the cylinder at a point about midway between the ends of the cylinder and should be long enough to permit movement of the support 60 along the slot 6I as far as it can go in both directions but not so long that it can touch the resistance element when the support is centrally located in the slot and the conductor is slack.

When the shaft 45, lead screw 56, cylinder 58 and the associated parts have been assembled as described, they are ready for assembly in the casing I Il. For this purpose the casing is supported in a fixture which holds it in a vertical position with the open end up. The shaft 45 is now rotated in a counter-clockwise direction relative to lead screw 56 until the support 60 comes to the end of the slot 6I in cylinder 58 and is then rotated approximately 90 degrees in a clockwise direction. The cylinder assembly is then brought to a position above the casing, with the shaft 45 extending downward, and is so oriented relative to the casing that the contact 54 on contact spring I3 is in alignment with the terminal I4. From this position the cylinder assembly is lowered into the casing, the shaft 45 entering its beaux ing in the bearing member 35, until the movement is arrested by the end of shaft 45 vengaging a stop with which the fixture is provided. The operator now inspects the contact 64 through the open end of the casing to make sure that itis properly centered on the end turn of the resistance element and adjusts the stop up or down if necessary.

The cap II is now inserted in the end of the The nut 59 and support 60 1 casing I9, with the lead screw projecting through the hole in the metal insert 41. The cap should be inserted with the terminal I6 a few degrees past terminal I4 in a clockwise direction to insure that the stop 52 will clear the support 50. The cap having been seated properly, the operator holds the lead screw with a screw driver and rotates the cap in a counter-clockwise direction far enough to bring the terminal I5 into alignment with terminals I4 and I5. The lead screw is now soldered to the insert 4'I and the casing with the parts assembled therein is removed from the iixture.

The next operation is the adjustment of the stop 52. In order to carry out this adjustment an ohmmeter is connected between the terminals I 5 and I4, the support 60 is held against the stop by applying a counter-clockwise torque to the shaft 45, and the stop is adjusted by rotating the cap I l in one direction or the other until the contact 64 is positioned on the last turn of the resistance wire which is soldered to the strip 3l, counting from the end of the resistance element. In this position of the Contact the ohmmeter will read substantially zero and will show an increased reading immediately responsive to clockwise rotation of the shaft 45. When the adjustment is completed the cap is temporarily held in position by a strip of tape to avoid any danger of its rotating relative to the casing.

The device may now be inspected and tested and if no defects are found the holes are drilled for the drive screws such as 10 and the screws are inserted. The cap II is thus securely xed in the casing.

The finished device may be mounted on a panel, such as il, as shown in Fig. l. A knob may be mounted on the shaft for rotating it, or the shaft may be rotated by gears, a flexible shaft, etc. depending on the particular situation in which the device is employed.

As mentioned before, the device is adapted to function either as a variable resistor or as a potentiometer. Assuming that it is to be used in the former capacity, the necessary circuit connections are made to the terminals I6 and I4. The circuit through the variable resistor extends from terminal I6 by way of rivet 48, spring 49 and contacts and 5I in parallel, disc 63, cylinder 58, conductor 66, slider or contact spring I3 and contact $4, resistance wire I9 of the resistance element, and strip 38 to terminal I4. With the support 59 resting against the stop 52, which is the counterclockwise stop, only about one-half turn or less of the resistance wire is included in the circuit and the resistance between the terminals is substantially zero.

When the shaft 45 is rotated in a clockwise direction, the cylinder 58 rotates the support 60 and nut 59 on the lead screw 56, causing the support BB to travel to the right along the slot 6I in the cylinder. The threads of the lead screw have the same pitch as the helical turns of the core of the resistance element, so that as the support 6i! is rotated the contact 4 travels along the resistance element and engages successive turns of the resistance wire. As the turns are cut into the circuit one after the other the resistance between the terminals I5 and I4 is proportionately increased, as will be understood.

When the shaft has been rotated somewhat less than 360 degrees the support 60 has moved far enough to the right so that it clears the stop 52 when it passes it. The rotation in a clockwise direction may continue for 3240 degrees more,

or for a total of 3600 degrees, when the support E will engage the clockwise stop and the rotation will cease. The clockwise stop is not shown, but it will be understood that it is formed in the end of the casing Ill by the abutment 44, Fig. 5. The contact 64 will now be in engagement with a turn of the resistance wire which is adjacent the turns soldered to the strip 38 associated with terminal I and the resistance between terminals I6 and I4 will be a maximum.

It will be noted that clockwise rotation of the shaft increases the resistance while counterclockwise rotation of the shaft decreases the resistance. For any particular angular setting of the shaft the resistance will always be the same regardless of whether the setting is approached in a clockwise or counter-clockwise direction. This desirable result is due to the fact that there is no lost motion in the drive between the cylinder 58 and the support 60, the part 65 of the contact spring I3 being effective to hold the support against the side of the slot 6I in the cylinder.

The operation of the device as a potentiometer will be understood from what has already been said and need not be explained in detail.

The invention having been described, that which is believed to be new and for which the protection of Letters Patent is desired will be pointed out in the appended claims.

We claim:

i. In a variable resistor, a cylindrical casing, a core formed into a multi-turn helix and fixed inside said casing, said core having a resistance wire helically wound thereon, an operating shaft having a bearing in one end of said casing, a threaded shaft fixed to the other end of said casing and extending into the casing in axial alignment with said first shaft, a bearing connecting said shafts whereby the said first shaft supports the adjacent end of said threaded shaft, a traveling nut carried on said threaded shaft, a contact supported on said nut and engaging said resistance wire, and an operating member for rotating said nut, said operating member being xed to said operating shaft and having a bearing on said threaded shaft adjacent the iixed end thereof.

2. In a variable resistor, a casing, a resistance element having the form of a compound helix fixed in said casing, an operating shaft having a bearing at one end of said casing, a threaded shaft disposed within said casing, said threaded shaft being fixed to the other end of the casing and having a bearing on said operating shaft, a slotted metallic cylinder disposed inside said helix, two discs of insulating material supporting said cylinder on said two shafts, respectively, said cylinder being xed to said operating shaft and rotatable on said threaded shaft, a contact support on said threaded shaft projecting through the slot in said cylinder, said support being rotatable on said threaded shaft and axially movable in said slot, and a contact mounted on said support and movable along the turns of said helix responsive to rotation of said operating shaft.

3. In a variable resistor, a resistance element in the form of a compound helix, a contact engaging said element, a support for said contact, means for rotating said support comprising a rotatable element having a slot therein spanning the turns of said helix, said support extending through said slot and the said slot being wider than said support, means including a lead screw for moving said support in said slot responsive to rotation of the support, whereby said contact `is causedto follow the turns "of said helix,l and resilient means for holding said support against one side of said slot, whereby the relation between the position of the contact on the resistance element and the extent of the rotation of said rotatable element is made independent of the direction of rotation.

4. In a variable resistor, a core formed into a multi-turn helix, said core having a resistance wire helically wound thereon, a cylindrical casing of plastic material molded around said helix, the turns of said helix and the turns of said wire being partly embedded in and held in spaced relation by said casing, a contact member engaging the said resistance wire inside said helix, means for rotating said contact member about the axis of the helix, and means for advancing said contact member along said axis responsive to such rotation.

5. In a variable resistor, a core formed into a multi-turn helix, said core having a resistance wire helically wound thereon, a cylindrical casing molded around said helix and effective to hold the turns thereof in spaced relation, said resistance wire having part of each turn embedded in said casing and part exposed inside said casing,-a movable contact associated with said resistance wire, means including an operating shaft supported on said casing for angularly advancing said contact to cause the sameto engage successive turns of said resistance wire, and means responsive to the angular advance of said contact for axially advancing the same to cause it to follow the turns of said helix.

6. In a variable resistor, an insulated metallic core formed into a multi-turn helix, said core having a bare resistance wire helically wound thereon in spaced turns, a cylindrical casing of plastic material molded around said helix and entering between the turns thereof to hold the turns in spaced relation, the turns of said resistance wire being embedded in the casing at the outside of the turns of said helix, whereby said wire turns are also held in spaced relation, a contact member engaging said resistance wire inside the turns of said helix, means supporting said contact member for rotary and axial movement, and means for operating said contact member to engage successive turns of said resistance wire.

'7. For use in the manufacture of variable resistors, a resistance element embedded in the interior wall of a cylindrical casing molded in situ of a heat setting plastic material, said helix comprising a core wound in helical form and a resistance wire helically wound on said core, and said casing acting to support the turns of said core and the turns of said resistance wire in spaced relation.

8. In a variable resistor, a molded cylindrical casing, a resistance element fixed in a said casing, said resistance element comprising a core wound into a helix and a resistance wire wound on said core in spaced turns, terminals for said winding embedded in said casing and connected to said winding at points adjacent to but short of the respective ends thereof, there being a plurality of turns at each end of the winding beyond the associated terminal, a movable contact in said casing, means for moving said contact along the turns of said helix and in contact with said resistance wire, and stops in said casing for preventing said contact from being moved past said terminals.

9. An article of manufacture, comprising a core ,75 Wound in a multi-turn helix, said core having a bare resistance wire Wound thereon, and a casing of plastic material molded around said helix, the material of said casing penetrating part way into the space between the helix turns and leaving a part of each resistance wire turn exposed within the helix, the remainder of each resistance wire turn being embedded in said material.

10. A11 article as claimed in claim 9, where in the plastic material adjacent the helix is reinforced by a textile fabric.

11. An article of manufacture, comprising an insulated metallic core formed into a multi-turn helix, a helical winding of bare resistance wire on said core, a layer of reinforcing material surrounding said helix, said layer having an internal helical ridge separating the turns of said helix, said layer being impregnated with plastic material, and a casing of plastic material molded around said helix and bonded to the said layer.

12. In the manufacture of potentiometers of the compound helix type having a rotatable and axially movable contact member and a casing with outside terminals for the helix and contact member, the method of calibration which consists in providing an angularly adjustable stop for said contact member adapted to arrest ,movement REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,152,913 Sagle Sept. 7, 1915 1,315,365 Hamm Sept. 9, 1919 1,505,600 Pickard Aug. 19, 1924 1,606,163 Douglas Nov. 9, 1926 1,630,874 Tuska May 3l, 1927 2,361,010 Cary et al Oct. 24, 1944 

